Several families of terrapin and turtle are found in lakes and rivers, while all true sea turtles belong to one of two families. The leather-backed sea turtle is the only member of the family Dermochelidae; all of the remaining sea turtles are placed in the family Chelonidae. The sea turtles possess flippers, but the freshwater species usually have only modified webbed feet.
Chelonia are members of the class Reptilia (see reptile) that include turtles, terrapins, and tortoises. Chelonians first appeared in the Triassic period, about 200 million years ago, and have changed little since. Their bodies are enclosed in a shell that overlaps a bony carapace above and a plastron below. Some chelonians have a soft or leathery shell: all lack teeth and have a horny beak.
The tortoises and terrapins have shells, typically with a horny layer covering a bony box derived from the backbone and ribs plus bony plates which originate in the skin. It is an effective armor, but heavy. Land tortoises are slow-moving, with legs adapted for weight-bearing; they sometimes have elephantine feet.
The most common chelonians are terrapins, amphibious freshwater forms with flatter shells than those of land tortoises. Their toes are webbed. Sea turtles are completely aquatic and eat mainly animal food, while land tortoises are largely vegetarian. No species of chelonian has teeth but deals with its food with horn-rimmed jaws, the front legs assisting in the tearing process.